Conventionally, ethylene and propylene are produced via steam cracking of paraffinic feedstocks including ethane, propane, naphtha and hydrowax. An alternative route to ethylene and propylene is an oxygenate-to-olefin (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can be converted into for instance methanol or dimethylether (DME), both of which are suitable feedstocks for an OTO process.
In an OTO process, an oxygenate such as methanol, ethanol or dimethylether is provided to a reaction zone of a reactor comprising a suitable conversion catalyst and converted into ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the oxygenate is converted into higher hydrocarbons including C4+ olefins, paraffins and carbonaceous deposits on the catalyst. The effluent from the reactor comprising the olefins, any unconverted oxygenates, some oxygenate byproducts and other reaction products such as water may then be treated to provide separate component streams. In case of a water-soluble oxygenate in the feedstock, such as for example methanol or ethanol, the greater part of the unreacted oxygenates can be separated from the reaction effluent, for instance by contacting with a cooled aqueous stream in a quench tower.
In order to increase the ethylene and propylene yield of the process, the C4+ olefins may be recycled to the reaction zone or alternatively further cracked in a dedicated olefin cracking zone to produce further ethylene and propylene.
Due to the high temperatures in the reaction zone and the acidity of the catalyst, a portion of the oxygenates such as methanol may unavoidably decompose thermally or catalytically into oxides of carbon, i.e. carbon monoxide and carbon dioxide in the gaseous form. The carbonaceous deposits on the catalyst can be removed by the periodic regeneration of the catalyst by heating it with an oxidising gas such as oxygen, in order to burn off the deposits.
Carbon dioxide generated during the OTO process is an acid gas which is thus present in the effluent from the reactor. In order to prevent contamination of the olefinic product and problems associated with the formation of solid carbon dioxide during the separation of the olefinic product into olefinic component streams, which may be carried out at cryogenic temperatures, carbon dioxide should be removed from the reaction effluent and from the gaseous effluent from the quench tower before separation into olefinic component streams. This is typically done by washing the gaseous effluent with a caustic solution in a caustic tower.
The gaseous effluent from the quench water tower in an OTO process typically still comprises small amounts of unconverted oxygenate and oxygenate byproduct. In case of a water-soluble oxygenate in the OTO feedstock, such as for example methanol or ethanol, the gaseous effluent comprises unconverted water-soluble oxygenate and some water-insoluble oxygenate byproduct, dimethylether in case methanol is used as feedstock and diethylether in case ethanol is used as feedstock. In case of a water-insoluble oxygenate in the feedstock, e.g. dimethylether or diethylether, typically water-soluble oxygenates are formed as byproduct (methanol in case of dimethylether as feedstock oxygenate and ethanol in case of diethylether as feedstock). Thus, the effluent of an OTO reactor and the gaseous effluent of a quench water tower in an OTO process comprise unconverted oxygenate and oxygenate byproduct of which at least the unconverted oxygenate or the oxygenate byproduct is a water-soluble oxygenate. Another source of water-soluble oxygenate in the gaseous olefinic stream in an OTO process may be the water-soluble oxygenate, typically methanol, that is often used for washing water-insoluble oxygenate (present as unconverted oxygenate or as oxygenate byproduct) from the quench water tower effluent in an OTO process. Dimethylether is typically present in such effluent as unconverted dimethylether in case of a feedstock comprising dimethylether or as byproduct in case of a feedstock comprising methanol.
Methanol or other water-soluble oxygenates in the gaseous quench tower effluent will not be removed by the wash treatment with a caustic solution in the caustic tower that is typically carried out to remove carbon dioxide and other acids. Since the presence of oxygenates in a propylene product that will be used for the manufacture of polypropylene will lead to undesired water production in the polypropylene manufacture, such oxygenates need to be removed from the OTO reaction effluent or from the propylene product stream recovered from such effluent. It is known to remove methanol or other water-soluble oxygenates by leading the propylene product stream over a guard bed prior to polypropylene manufacture.
Alternatively, the water-soluble oxygenates are washed from the gaseous effluent from the quench tower by means of a water wash step upstream of the caustic tower.